Molecular sieves are used as catalysts in various hydrocarbon conversion processes. In most processes the molecular sieves are formed into shaped articles such as spheres, extrudates, etc. It has been found that some of the processes are diffusionally limited and thus the molecular sieve on the interior of the shaped articles are not utilized in the reaction. Alternatively, owing to the long diffusion path, compounds can undergo further reactions leading to the formation of undesirable byproducts. Further, these shaped particles are formed using some catalytically inert binder and thus a pure molecular sieve is not available to catalyze the reaction.
There are a number of references which disclose layered compositions. For example, U.S. Pat. No. 4,283,583 discloses a coated zeolite catalyst consisting of an inert core and an outer coating comprising an active catalytic zeolite material. The catalyst is prepared by wetting the inner core partially drying and then contacting the core with a zeolite powder. U.S. Pat. No. 4,482,774 discloses a composite zeolite having a crystalline silica polymorph as the core material and a modified silica overlayer which has substantially the same crystalline structure. The overlayer is formed by adding preformed particles of the silica core into a crystallization gel at crystallization conditions thereby crystallizing the zeolite onto the core. U.S. Pat. No. 4,088,605 discloses growing a substantially aluminum free shell onto an aluminum containing zeolite. U.S. Pat. No. 5,895,769 discloses depositing a polycrystalline zeolite onto a porous substrate. U.S. Pat. No. 5,935,889 discloses preparing catalyst particles by coating core particles with an atomized slurry containing a coating material. Finally, U.S. Pat. No. 6,013,851 discloses a core zeolite having deposited thereon a surface layer where the surface layer has a higher Si/Al ratio than the core.
In contrast to these references, applicants have developed a process which grows a molecular sieve layer onto an inner core. The process involves providing a slurry comprising inner core particles and then adding to the slurry reactive sources (nutrient(s)) of the framework element(s) of the molecular sieve in order to form crystals of the molecular sieve. As the crystals form, they agglomerate onto the inner core and after sufficient time form the desired layer thickness. A preferred procedure involves first adding the nutrient(s) intermittently to form crystals and then adding the nutrients continuously to grow the crystals that have agglomerated onto the inner core.